The present invention relates to an AC/DC converter for converting a commercially available AC power source to a desired DC output and, more particularly, to an AC/DC converter advantageously usable as an AC adapter for household electrical appliances including a notebook type personal computer.
An AC/DC converter for the above application is usually implemented by a switching regulator system and includes an electromagnetic transformer and a switching circuit operable at about 100 kHz by high frequency noise regulation (VCCI, FCC). Japanese Patent Laid-Open Publication No. 6-245548 teaches an inverter circuit for outputting a high voltage adapted for the liquid crystal backlight of, e.g., a notebook type personal computer. The inverter circuit taught in this document uses a piezoelectric transformer which is small size and light weight and has a high boosting ratio. Japanese Patent Laid-Open Publication No. 7-39144 discloses a DC/DC converter using a piezoelectric transformer operable in a thickness-extensional vibration mode and resonating in a megahertz frequency band.
A conventional AC/DC converter uses an electromagnetic transformer for outputting a desired DC voltage. The electromagnetic transformer is required to have its primary winding and secondary winding spaced by a prescribed distance for insulation from the safety standpoint. Such a distance for insulation obstructs the miniaturization of the transformer. By contrast, a piezoelectric transformer is formed of ceramics and therefore small size, incombustible, highly isolative, and high frequency, high power efficiency, boosting ratio, and high power density. With these features, a piezoelectric transformer is implemented as an inverter circuit for outputting a high voltage adapted for a liquid crystal backlight or as a DC/DC converter. On the other hand, for a high voltage inverter, it is a common practice to use a Rosen type piezoelectric transformer. This type of transformer outputs a current of only several milliamperes and is not applicable to an AC/DC converter required to output a current of several amperes. The piezoelectric transformer operable in the thickness-extensional vibration mode and used as a DC/DC converter operates in the megahertz frequency band uses an electrolytic capacitor for a smoothing purpose. This kind of transformer is not feasible for an AC/DC converter due to the electrolytic capacitor and because the AC/DC converter must operate in substantially the 100 kHz frequency band. For the above reasons, applying a piezoelectric transformer to an AC/DC converter which transforms a commercial AC power source to a DC output has not been considered.